Versatile Motion Control of a Magnetic Skyrmion Pair with Spin Transfer Torques
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Versatile Motion Control of a Magnetic Skyrmion Pair with Spin Transfer Torques N. J. Kim, S. P. Kang, C. Lee, H. G. Yoon, H. Y. Kwon and C. Won∗ Department of Physics, Kyung Hee University, Seoul 02447, Korea (Received 24 November 2019; revised 21 January 2020; accepted 19 February 2020) We studied the motion of a skyrmion pair under the action of independently controllable spin transfer torques. A skyrmion pair made of two coupled skyrmions with opposite skyrmion numbers moves along the direction of the current flow when the currents flowing in the two skyrmions, are identical while it moves perpendicular to the direction of current flow where the currents flow in opposite directions. Manipulating all the degrees of freedom for the strengths of both currents enables the directional motion of the skyrmion pair. We used the Thiele equation to investigate how the strengths of the currents are related with the motion of the skyrmion, and we obtained a tensor that provided the relation between the currents and the force that binds the two skyrmions, and we were able to realize a designed motion. We performed a dynamic simulation of the motion of the skyrmion pair and compared the results with those calculated using the Thiele equation. Keywords: Magnetic Skyrmion, Spin Transfer Torque, Skyrmion Hall effect DOI: 10.3938/jkps.77.869
I. INTRODUCTION Magnetic skyrmions have emerged as candidates for use as information units or carriers in spintronics due to their stability and mobility, and their properties have been intensively investigated in recent years [1– 11]. A skyrmion is stable because its magnetic structure comprises spatially twisted spins characterized by the skyrmion number. In addition to its stable structure, it has high mobility under spin transfer torques (STTs) because its motion arises from the gyration of spin under the torque [1,2]. These advantages have led to intensive research focused on the manipulation of the dynamic properties of skyrmions [3,4]. Applying a STT by using electric current is one way to move a skyrmion. However, a skyrmion does not move simply along the direction of the current, but shows a tilted movement not only due to the damping effect but also because the gyration involves spin dynamics. The gyration of the magnetization is the origin of the tilted movement, as a recent work using ferrimagnets reveals that no gyration of magnetization induced such an effect [12]. This is known as the skyrmion Hall effect. Various concepts such as skyrmion pair, ferrimagnetic skyrmion, antiferromagnetic skyrmion, and synthetic antiferromagnetic skyrmion, were proposed to manipulate the skyrmion Hall effect [12–21]. A skyrmion pair consists of two skyrmions that have skyrmion numbers opposite to each other, so that the total skyrmion number is ∗ E-mail:
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pISSN:0374-4884/eISSN:1976-8524
zero. The antiferromagnetic interlayer coupling between two layers can be used to bind two skyrmions with opposite skyrmion numbers. The spins in each layer align in anti-parallel direction, which makes
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